The present invention concerns a novel apparatus and process for enabling selected tuning of the tuned coil in an enclosure that is implanted in a patient.
In U.S. Pat. No. 4,361,153, issued Nov. 30, 1982 and assigned to the assignee of the present invention, there is disclosed the use of a resonant impedance modulated transponder, in a device implanted in the patient, to modulate the phase of a reflected magnetic signal that is the product of a magnetic carrier imposed from outside of the body. In this manner, information is transmitted from a fixed internal implant to a positionable external telemetry unit. A relative high energy magnetic field at a carrier frequency is established by a transmitter in the external unit. The field permeates the skin, underlying tissue and case of the implant and induces a signal in a resonant, impedance modulated transponder in the implant tuned to the carrier frequency. A second field is reradiated or reflected at the carrier frequency by the resonant transponder. The transponder's impedance is varied in accordance with a modulation input signal, causing a shift in the phase angle and amplitude of the transponder's contribution to the composite reflected signal, thereby resulting in a proportional phase and amplitude shift in the composite reflected signal. The composite reflected signal is picked up and demodulated by a phase shift detector in the external telemetry unit.
While certain telemetry units generate a carrier signal at a first frequency, for example 16 kilohertz, other telemetry units may generate a carrier signal at a different frequency, for example, 64 kilohertz. Since the higher frequency allows more sampling of the data per second, it is sometimes found desirable to use the higher frequency carrier signal.
If the resonant frequency of the transponder in the implant is tuned to a lower frequency, for example, 16 kilohertz and a higher frequency carrier signal, for example, 64 kilohertz, is utilized, the detected phase and amplitude shift in the composite reflected signal is relatively poor.
It is, therefore, an object of the present invention to provide a system for enabling the changing of the tuned frequency, so that it can correspond to the selected carrier frequency generated by the external unit.
It is also an object of the present invention to provide a system for selectively tuning a tank circuit that is efficient to manufacture and reliable in use.
Further objects and advantages of the present invention will become apparent as the description proceeds.